WEDNESDAY, Feb. 25, 2015 (HealthDay News) -- Young children are at increased risk of becoming seriously ill with pneumonia -- but unlike in years past, the cause is usually a respiratory virus, a large U.S. study finds.

The researchers found that 66 percent of pneumonia cases in the more than 2,000 children in the study were caused by viruses alone. Just 8 percent had solely bacterial causes, and 7 percent were known to be caused by both bacteria and viruses. And, those infections can end up being serious, the study authors said.

"Pneumonia is one of the most common causes of hospitalizations among children," said the study's lead author, Dr. Seema Jain, a researcher at the U.S. Centers for Disease Control and Prevention (CDC).

Pneumonia is a general term for an infection of the lungs, and it can be caused by a range of viruses, bacteria or even fungi. The disease is often thought of as a problem affecting elderly adults -- especially when it's severe enough to warrant a hospital stay.

However, children -- especially younger children -- are also at increased risk, according to Jain. About 16 children out of every 10,000 get pneumonia every year. In youngsters under 2 years old, that number jumps to 62, according to the study published in the Feb. 26 issue of the New England Journal of Medicine.

But while it's well known that childhood pneumonia is a public health concern, these latest findings help to better "quantify" the problem, Jain said.

For the study, her team tested body fluid samples from over 2,200 children treated for pneumonia at three U.S. children's hospitals. The investigators found that nearly three-quarters of the children had viral infections -- either alone or in combination with a bacterial infection. The infections included a range of viruses that often cause congestion and other cold symptoms, but can also lead to more-severe infections such as pneumonia.

The most commonly detected virus was respiratory syncytial virus, or RSV. According to the CDC, almost all children contract an RSV infection by their second birthday. Anywhere from 25 percent to 40 percent develop either bronchiolitis (inflammation in the small airways of the lungs) or pneumonia the first time they're exposed to the virus.

On the other hand, Jain's team found, only 15 percent of children in the study had evidence of a bacterial infection (alone or in combination with a viral infection).

Thirty or 40 years ago, the opposite pattern would have been true, according to Dr. William Schaffner, an infectious disease specialist at Vanderbilt University, in Nashville, Tenn.

"Bacterial causes would have been predominant," said Schaffner, who is also a spokesperson for the Infectious Diseases Society of America. "What's changed is that now we have vaccines that prevent bacterial pneumonia."

Schaffner noted that two vaccines target once-major causes of bacterial pneumonia: the Hib vaccine, which protects against Haemophilus influenzae type B; and the pneumococcal vaccine, which wards off a group of bacteria that cause pneumonia and serious infections of the bloodstream and brain.

Both he and Jain said the findings underscore the importance of keeping children up-to-date with recommended vaccinations. According to the CDC, some other standard vaccines -- including the shots against measles, whooping cough and chickenpox -- can also curb children's risk of pneumonia.

But the findings also have implications for treatment, Schaffner pointed out. If most cases of child pneumonia are not caused by bacteria, then antibiotics (which are useless against viruses) will often be unnecessary.

Dr. Adriana Cadilla, a pediatric infectious disease specialist at Miami Children's Hospital agreed. "Studies like this support what we've been trying to practice limiting unnecessary antibiotic use," she said.

All of the children in this study had fluid samples tested for pathogens, but in the real world that does not always happen. Jain said that when a child is sick with pneumonia, parents should feel free to ask whether any testing was done to pinpoint the cause.

But right now, Jain and her colleagues write, there is a need for better tests -- ones that are more sensitive, cheaper and faster.

Jain pointed to another critical need: Vaccines and drugs against RSV and other major viral causes of pneumonia.

Schaffner concurred. "This study helps shows us what the next big target should be," he said. "And researchers are working on vaccines against RSV, as we speak. In 10 years, maybe we'll see that the incidence of RSV pneumonia is going down."